1. Field of the Invention
The present invention relates to an aberration correction apparatus and method for correcting aberration produced in a light beam of an optical system.
2. Description of the Related Art
Optical discs such as CD (Compact Disc) and DVD (Digital Video Disc or Digital Versatile Disc) are known as information recording media on which information is optically recorded or reproduced. In addition, various types of optical discs are now under development, such as an optical disc dedicated to reproduction, a write-once optical disc on which information can be additionally recorded, and a rewritable optical disc on which information can be erased and recorded again.
Also, research and development has advanced for optical pickup devices and information recording/reproducing apparatus which can be adapted to higher density discs, with the trend of increasing the density of optical discs. In addition, research and development has also advanced for so-called compatible optical pickup devices and information recording/reproducing apparatus which can utilize different types of optical discs.
It is contemplated that the numerical aperture (NA) of an objective lens provided in an optical pickup device is increased to irradiate an optical disc with a light beam having a smaller irradiation diameter in order to support the higher density optical discs. Also, a light beam having a shorter wavelength is used to address the higher density discs.
However, an increase in the numerical aperture NA of an objective lens or employment of a light beam having a shorter wavelength results in a larger influence of aberration on the light beam by the optical disc, thereby making it difficult to improve the accuracy of information recording and information reproduction.
For example, the amount of birefringence, which depends on the incident angle, will have a larger distribution width on the pupil plane of the optical disc since a larger numerical aperture NA of an objective lens results in a wider incident angle range of a light beam for an optical disc. This causes a problem of increased aberration influence resulting from the birefringence. Also, aberration due to an error or variance in the thickness of a cover layer for protecting a recording surface of the disc exerts a larger influence.
To reduce the influence of the aberration as mentioned, a pickup device provided with a liquid crystal device for correcting the aberration has been previously proposed. Such an aberration correction device is disclosed, for example, in Japanese Patent Application Kokai No. H10-269611. The aberration correction device has a plurality of concentrically formed phase adjustment portions, and applies each electrode with a predetermined voltage to adjust an orientation state of the liquid crystal to correct aberration produced in a light beam. However, the aberration correction device configured as described has a problem that it can correct only wavefront aberration which has a particular distribution shape. Also, for correcting large aberration which may exceed the wavelength of a light source, a thicker liquid crystal must be provided to apply a larger voltage. Further, a large number of phase adjustment portions must be provided for accommodating the wavefront aberration of various magnitudes. An increase in the number of electrodes would result in requirements for a very large number of electrodes and complicated wiring. Consequently, this would constitute a hindrance in the trend of pursuing higher performance of the aberration correction device, including a reduction in size and thickness and a higher speed. Also, the thicker liquid crystal causes a problem of degrading the frequency response of the aberration correction device.
Another aberration correction method uses a hologram element which deflects light transmitting therethrough to produce a phase difference in the light. However, the hologram element cannot be turned on/off as required. In addition, similar to the foregoing, the hologram element has a problem in that it can correct only wavefront aberration that has a particular distribution shape. The hologram element is further disadvantageous in that it is difficult and expensive to manufacture.
The present invention has been made in view of the problems mentioned above, and the object of the present invention is to provide a compact and high performance aberration correction unit which is capable of correcting aberration over an entire range of an optical path diameter, with the ability of correcting large aberration as well. It is also an object of the present invention to provide an optical pickup device and an aberration correction apparatus which have the aberration correction unit.
It is another object of the present invention to provide a compact and high-performance aberration correction apparatus and a method which are capable of correcting aberration having a variety of shapes and capable of correcting large aberration as well.
To achieve the object, according to one aspect of the present invention, there is provided an aberration correction unit disposed in an optical path of an optical system for irradiating a recording medium with a light beam for correcting aberration produced in the light beam, the aberration correction unit comprises: a first electrode and a second electrode being opposed to each other; and a liquid crystal disposed between the first electrode and the second electrode for applying a voltage to produce a change in phase to light passing therethrough, wherein the first electrode and the second electrode are formed in a shape such that the liquid crystal produces an effect of holographic diffraction grating by applying voltages to the first electrode and the second electrode.
According to another aspect of the present invention, there is provided an optical pickup device having the aberration correction unit, which comprises: a light source for emitting the light beam; an optical system for guiding the light beam; and an optical detector for detecting a light beam which has been reflected by the recording medium and transmitted the aberration correction unit.
According to another aspect of the present invention, there is provided an aberration correction apparatus having the optical pickup device, which comprises: voltage applying means for applying voltages to the first electrode and the second electrode of the aberration correction unit such that the liquid crystal produces an effect of holographic diffraction grating.
According to another aspect of the present invention, there is provided an aberration correction device for correcting aberration produced in a light beam of an optical system for guiding the light beam, which comprises: a first electrode layer and a second electrode layer being opposed to each other; and a liquid crystal disposed between the first electrode layer and the second electrode layer for producing a change in phase in a light beam passing therethrough in accordance with voltages applied to the first electrode layer and the second electrode layer, wherein the first electrode layer is divided into a plurality of phase adjustment portions which are electrically isolated each other, each of the plurality of phase adjustment portions including a pair of electrodes arranged in such a form that produces an electric field distribution for correcting the aberration by applying predetermined voltages thereto.
According to another aspect of the present invention, there is provided an aberration correction apparatus having the aberration correction device, which comprises: a determination section for determining each aberration amount of the light beam in a region corresponding to each of the plurality of phase adjustment portions; a voltage applying section for applying each of the plurality of phase adjustment portions with voltages corresponding to each of the determined aberration amounts; a selecting section for selecting a phase adjustment region including one or more consecutive phase adjustment portions based on each of the determined aberration amounts; and a controller for conducting a control for changing a phase-adjustment amount in the phase adjustment region to a phase amount corresponding to an integer multiple of the wavelength of the light beam.
According to another aspect of the present invention, there is provided an aberration correction apparatus for correcting aberration produced in a light beam of an optical system for guiding the light beam, which comprises: an aberration correction device having a plurality of phase adjustment portions for producing a phase change in the light beam by an voltage applied thereto; a phase adjustment section for applying voltage to each of the plurality of phase adjustment portions to adjust the phase; a determination section for determining whether a phase-adjustment amount of each the phase adjustment portion exceeds a predetermined limit adjustment amount; and a controller for changing the phase-adjustment amount of each the phase adjustment portion to a value resulting from a subtraction of a phase amount corresponding to an integer multiple of the wavelength of the light beam from the phase-adjustment amount on the basis of the result of determination made by the determination section.
According to another aspect of the present invention, there is provided a method of correcting aberration for an aberration correction device having a plurality of phase adjustment portions for producing a phase change in a light passing through the aberration correction device to adjust the phase, which comprises the steps of: applying a voltage to each of the plurality of phase adjustment portions to adjust the phase; determining whether a phase-adjustment amount of the phase adjustment portion exceeds a limit adjustment amount; and changing the phase-adjustment amount of each the phase adjustment portion to a value resulting from a subtraction of a phase amount corresponding to an integer multiple of the wavelength of the light beam from the phase-adjustment amount on the basis of the result of the determination in the determining step.